Solar Smackdown: Photovoltaics vs. Photosynthesis

Photonics.comMay 2011
PHOENIX, May 16, 2011 — Because photosynthesis and photovoltaics harvest energy from the sun in distinctly different ways and produce different fuels, it is difficult to compare energy conversion efficiency.

"In order to make meaningful comparisons between photosynthesis (which provides stored chemical potential) and photovoltaic technology (which provides instantaneous electrical power), we considered photovoltaic driven water electrolysis to yield hydrogen using existing technology as an example of artificial photosynthesis," explained Thomas Moore, director of the Center for Bioenergy and Photosynthesis at Arizona State University and co-author of the study published in the journal Science.

"The hydrogen produced by the artificial system is thermodynamically equivalent to the sugar produced by photosynthesis. The take-home from this point is that the artificial system outperforms the natural one, but on the basis of potential for efficient solar energy conversion as measured by the land area required for a given energy output, both technological and biological processes could in principle offer similar outcomes."

Photosynthesis or photovoltaics? Which is more efficient at harvesting the sun's energy, plants or solar cells? This salient question and an answer are the subject of an article published in the May 13 issue of the journal Science. (Image: Bob Blankenship)
Photovoltaic technology uses fundamental principles combined with advances in materials to achieve record efficiencies of solar-to-electrical power conversion and thereby hydrogen production from water electrolysis.

Photosynthesis, on the other hand, originated in an environment where it was selected, as it provided early life forms with a means of self-contained energy production. However, as with many evolutionary adaptations, it is far from a perfect system for the production of energy, and it is certainly not optimal for providing solar-derived fuel to support human activities and economies.

All natural photosynthetic organisms contain light-gathering antenna systems in which specialized pigments (typically several hundred molecules) collect energy and transfer it to a reaction center where photochemistry takes place.

With so many pigments absorbing light energy, the capacity of the photosynthetic apparatus to process the energy is quickly exceeded. In leaves in full sun, up to 80 percent of the absorbed energy must be dumped to avoid its diversion into toxic chemical reactions that could damage or even kill the plant.

Modern agriculture has pushed photosynthesis about as far as it can go based on incremental improvements such as selection for high-yield crops, land use improvements, use of modern fertilizers, water use, pesticides to control pests and, in short, the green revolution and all that it entails.

Professor Thomas Moore is director of the Center for Bioenergy and Photosynthesis at Arizona State University. (Image: Tom Story)
"We have identified many of the important inefficiencies that arise from the basic design of photosynthesis and have suggested ways to re-engineer photosynthesis to improve its ability to meet human energy needs," explains Moore, a Regents' Professor in the department of chemistry and biochemistry in the College of Liberal Arts and Sciences.

"These improvements to photosynthesis go beyond the incremental steps practiced since agriculture began thousands of years ago. At the end, we allude to the use of synthetic biology to bring the knowledge and experience from fundamental studies in physics and artificial photosynthesis to photosynthesis in a combination of biology with technology to meet human energy needs."

Operating at approximately 133 TW, photosynthesis powers the biosphere and thereby life on Earth. Currently, human activity appropriates about 24 percent of photosynthetic net primary production (NPP) to support the US gross domestic product and nutrition.

The cost to the biosphere of "our cut" of NPP is driving several Earth systems irreversibly across boundaries that were established over geological time scales, said Moore. Earth systems affected include the nitrogen cycle, carbon cycle, fresh water, land use and an increase in the rate of biodiversity loss. In other words, photosynthetic energy flow is currently booked (almost certainly overbooked) for biosphere services including food and limited bioenergy production for human use. As a consequence, there are no reserves of photosynthetic capacity to provide increasing amounts of biofuel for growing our GDP and food for the ever-increasing human population. Indeed, when such demands are made, the capacity comes at the further peril of biosphere services.

"Fortunately, the efficiency of photosynthetic NPP could be dramatically improved to meet human needs — the 133 terawatts increased to about 150 terawatts with minimum additional impact on Earth systems," explained Moore. "I'm thinking about selected photosynthetic systems in which rational design, based on the principles demonstrated in artificial systems, could be used to optimize solar-to-biofuel conversion efficiencies to meet particular needs.

“Such photosynthetic systems would be 'living' in that they would retain key features of living cells, including self-assembly, repair, replication and the use of Earth-abundant materials — features that I think are essential to scale and match sustainable energy production to local needs but that remain elusive to nonliving, human engineered constructs," Moore said.